A cladding-pumped fiber laser typically comprises a single-mode fiber core disposed within a relatively large multimode cladding. The cladding is surrounded by a further layer to prevent radiation from propagating out of the cladding. The fiber is positioned between two mirrors adjacent to its end faces which define the laser cavity.
Light from a pump laser, such as a laser diode, is injected into the end or the side of the cladding. The geometry and refractive indices of the core and cladding arrangement are such that a substantial amount of the radiation propagating in the cladding is coupled into the single mode core. This is advantageous since radiation can be coupled into the multimode cladding without the high tolerances typically required for coupling light directly into a single mode core. See, Senior, Optical Fiber Communications: Principles and Practice, (2d. ed., Prentice Hall, 1992), pp. 347-349. This reference, as well as all others mentioned in this specification, are incorporated herein by reference.
The core is doped with ionized rare-earth elements, which are the active lasing elements. The active lasing elements absorb photons delivered by the pump laser. Photons are then emitted by the active lasing elements at a wavelength characteristic of the particular dopant species.
Presently existing high power cladding-pumped fiber lasers are typically 15 to 60 meters in length. Such lengths are required for full absorption of pump laser power. For high-power transmission, the laser exhibits a number of longitudinal operating modes. The prior art high power cladding-pumped lasers suffer from a number of drawbacks related to their length and the plurality of longitudinal modes operating, as discussed below.
One such drawback is a phenomena known as mode beating noise. Mode beating noise results from the mixing of the numerous longitudinal operating modes. It is desirable to limit the frequency range in which such noise occurs to frequencies above 500 MHz. To do so requires a laser oscillator about 20 cm in length, much shorter than the typical 15-60 meter cavity.
Further, to transmit high power without Stimulated Brillioun Scattering (SBS) occurring, it is desirable to have the plurality of laser modes spaced by 25 MHz or greater. This corresponds to a cavity length of 4 meters or less, again, significantly shorter than the currently available 15-60 meter cladding-pumped fiber lasers.
The relatively long prior art cladding-pumped fiber lasers typically have weak output couplers. These weak output couplers typically have an output reflectivity, R, of about 4 percent, which makes them sensitive to feedback from other sources. A short oscillator with a much more highly reflecting output coupler, such as one having an R greater than 50% and as high as 90%, should be much less feedback sensitive.
High gain cladding-pumped fiber lasers tend to readily self mode-lock, or at least experience a significant amount of round-trip noise. The frequency of the noise may be controlled by varying the cavity length of the oscillator. While the exact mechanism of mode locking is not known, the amount of birefringence or polarization rotation in a long cavity may be play an role in causing the phenoma. A short oscillator cavity should significantly reduce these effects, and hence the tendency to mode-lock.
Typical cladding-pumped fiber lasers have a limited operating wavelength range. The wavelength of operation of a quasi-three level laser such as Yb.sup.3+ is a function of the amount of inversion averaged over the entire laser length. If a short laser oscillator is located at the highly-pumped front end, the amount of inversion is higher and the laser can therefore operate at shorter wavelengths closer to the three-level transition. This would extend the range of wavelengths available from cladding-pumped fiber lasers.
Thus, there is a need for an improved cladding-pumped laser structure which addresses the aforementioned shortcomings.